The present invention relates to an optical current transformer to measure an electric current using Faraday effect of light, and more particularly to an optical current transformer which is capable of accurately measuring an electric current without influence of an external electric current except for the electric current which is to be measured or capable of measuring a large electric current.
This application is based on Japanese Patent Application No. 8-229837, filed Aug. 30, 1996, the content of which is incorporated herein by reference.
Hitherto, an apparatus of a type using light, that is an optical current transformer has been developed to measure an electric current for a power transmission and substation system. The optical current transformer is arranged in such a way that a block made of lead glass or the like to serve as a sensor is disposed adjacent to a conductor through which an electric current to be measured flows. Moreover, linearly polarized light is allowed to pass through the sensor to measure the angle of rotation of the Faraday effect generated because of a magnetic field created by the electric current. An example of the conventional optical current transformer is shown in FIG. 1.
FIG. 1 shows an example of an optical current transformer for a gas insulated switchgear. As shown in FIG. 1, a conductor 2 allowing a high voltage electric current to flow therein is included in a tank 1, the voltage of which is the ground potential. The conductor 2 permits an electric current to flow in a direction perpendicular to the drawing sheet. A block-shape sensor 3 made of lead glass or the like is disposed to surround the conductor 2, the sensor 3 being fixed by a fixing member 4. To enable the conductor 2 to permit a high voltage electric current to flow, the fixing member 4 of the sensor 3 is attached to the tank 1 through an insulating pipe 5 so that the fixing member 4 of the sensor 3 is insulated from the tank 1. A box 6 including an optical system is attached below the tank 1. The box 6 includes an coupling optical system 7, a light transmitting fiber 8 and two light receiving fibers 9a and 9b.
The coupling optical system 7 comprises a lens 7a and a polarizer 7b. The fibers 8, 9a and 9b are optically connected to the sensor 3 through the coupling optical system 7. The light transmitting fiber 8 transmits a measuring light generated by a light source (not shown) to the sensor 3 through the coupling optical system 7. The light receiving fibers 9a and 9b respectively receive light which has been, by the coupling optical system 7, divided into components linearly polarized into two directions perpendicular to each other to transmit the light components to a signal processing system (not shown).
The optical current transformer having the above-mentioned structure shown in FIG. 1 is able to measure an electric current which flows in the conductor 2 in accordance with the following principle.
Initially, light emitted from the light source (not shown) is allowed to pass through the light transmitting fiber 8, and then introduced into the coupling optical system 7. Light is, in the coupling optical system 7, formed into a linearly polarized light 10a in the form of a substantially parallel beam to propagate through a space in the insulating pipe 5. Thus, the linearly polarized light 10a is incident on the sensor 3 made of the lead glass, and then circulated around the conductor 2 in such a way that the linearly polarized light 10a is reflected repeatedly in the sensor 3. Then, the linearly polarized light 10a is emitted from the sensor 3. During this propagation, the polarization plane of light which passes through the sensor 3 is rotated by an angle corresponding to the level of an electric current due to the Faraday effect induced by the electric current which flows in the conductor 2.
The light emitted from the sensor 3 is formed into a linearly polarized light 10b which propagates through the space, and then again is incident on the coupling optical system 7 so that light is divided into two components polarized linearly in the two directions perpendicular to each other and then respectively is incident on the two light receiving fibers 9a and 9b.
Hereinafter, the light which is emitted from the polarizer 7b and transmitted to the light receiving fiber 9a or 9b through the sensor 3 is called the polarized measuring light. In FIG. 1, the polarized measuring light includes the linearly polarized light 10a, the light transmitted in the sensor 3, and the linearly polarized light 10b. The incident light in the form of the two components is processed by the signal processing system so that the angle of rotation, that is, the level of the electric current which flows in the conductor 2 is measured. Since the members and operations of the coupling optical system 7 and the signal processing system are known facts, they are omitted from detailed descriptions.
The apparatus for measuring an electric current for the power transmission and substation system must satisfy the following requirements:
(1) The size can be reduced and the structure can be simplified. PA1 (2) The apparatus is able to always precisely measure the electric current without influence from an external electric current generated by an adjacent conductor except for the conductor which is to be measured. PA1 (3) The apparatus is also able to measure a large electric current. PA1 (4) The cost of the apparatus must be reduced to be widely used. PA1 a light source for generating a polarized measuring light; PA1 an optical fiber wound around a conductor through which an electric current to be measured flows in order to circulate the measuring light around the conductor, the polarization plane of the light which propagates through the optical fiber being rotated by a magnetic field generated by the electric current; and PA1 means for detecting the level of the electric current by detecting an angle of rotation of the polarization plane of the light emitted from the optical fiber.
However, the conventional optical current transformer shown in FIG. 1 has a problem that the optical current transformer is affected by an external electric current. Therefore, the above-mentioned optical current transformer cannot always precisely measure an electric current. The optical path in the sensor 3 of the optical current transformer shown in FIG. 1 is not formed into a complete closed loop because a surplus optical path indicated by symbol L exists. Therefore, light is, in this surplus optical path, affected by an external magnetic field created by an external electric current or the like. As a result, the electric current, which flows in the conductor 2 cannot accurately be measured. The external electric current may be a sheath current which flows in the tank 1, currents in other phases in a case where the currents in respective phases flow in respective tanks, or current which flow conductors in other phases in a case where the conductors in all phases are included in the same tank 1.
Since the sensor 3 has excellent sensitivity (because lead glass has a large Verdet constant), the conventional optical current transformer suitable to measure a small electric current. However, a large electric current cannot easily be measured.
Moreover, the conventional apparatus using the block-shape sensor 3 cannot be formed into a small and simple structure. Therefore, the cost, which is required to be reduced for the purpose of realizing wide use of the apparatus, cannot easily be reduced.